As prior art inspection apparatuses in connection with the present invention, an apparatus using a scanning electron microscope (SEM) has already been launched on the market. This apparatus is designed in such a way that an electron beam converged slenderly is subjected to raster scanning at a raster width having an extremely small interval, forming a SEM image by detecting the secondary electron emitted from the object of inspection upon scanning, and extracting a defect by comparing the SEM image at the same position of different dice.
Further, many proposals have been made so far that a throughput can be improved by using plural electron beams, that is, multi-beams. The proposals disclosed are directed primarily to the way of forming the multi-beams and to the way of detecting the multi-beams. No proposal, however, have been yet made as to an apparatus that has completed a whole system for a defect inspection apparatus.
In order to detect a defect of a mask pattern for use in manufacturing semiconductor devices or a pattern formed on a semiconductor wafer, a scanning electron microscope has been used. The scanning electron microscope requires a long time for inspection of a whole sample because the surface of the sample is scanned with one electron beam converged slenderly and the secondary electrons emitted from the sample are to be detected. In order to solve these problems, it has been proposed that the electrons from a plurality of electron sources are focused on the plane of a sample through a decelerating electron field lens and scanned to deflect the secondary electrons emitted from the surface of the sample by means of a Wien's filter, thereby guiding the deflected secondary electrons to a plurality of detectors (Japanese Journal of Applied Physics, Vol. 28, No. 10, October, 1989, pp. 2058-2064).
For an apparatus for exposing a pattern of a semiconductor circuit or the like to the surface of a sample such as a semiconductor wafer or the like or for inspecting a pattern formed on the surface of such a sample by irradiating the surface of the sample with charged particle beams, such as electron beams or the like, or for an apparatus for subjecting the sample to very high precision processing by irradiating it with the charged particle beams, a stage is used that can align the sample in vacuum with high degree of precision.
When such a stage requires alignment at a very high level of precision, the stage uses a structure that it is supported in a non-contact way by means of a hydrostatic bearing. In this configuration, the vacuum level in a vacuum chamber can be sustained by forming a differential exhaust mechanism for discharging high pressure gases within the range of the hydrostatic bearing so as to prevent the high pressure gases to be supplied from the hydrostatic bearing from being emitted directly into the vacuum chamber.
An example of such a conventional stage is shown in FIGS. 18A and 18B. In the configuration as shown in FIGS. 18A and 18B, a top end portion of a lens barrel 2001 of a charged beam apparatus for irradiating a sample with charged beams, that is, a charged beam irradiation portion 2002, is mounted on a housing 2008 constituting a vacuum chamber C. The inside of the lens barrel is made in a vacuum state by discharging the air with a vacuum line 2010, and the vacuum chamber C is made in a vacuum state by discharging the air with a vacuum line 2011. Charged beams are irradiated from the top end portion 2002 of the lens barrel 2001 onto the sample S such as a wafer, etc. disposed thereunder.
The sample S is detachably held on a sample table 2004 by conventional means. The sample table 2004 is mounted on top surface of a Y-directionally movable portion 2005 of an XY stage (hereinafter referred to as “the stage”) 2003. The Y-directionally movable portion 2005 is slidably mounted on an X-directionally movable portion 2006, and the X-directionally movable portion 2006 is slidably mounted on a stage table 2007.
The Y-directionally movable portion 2005 is installed with a plurality of hydrostatic bearings 2009a on the surface (the left- and right-hand surfaces and the bottom surface in FIG. 18A) opposite to a guide surface 6a of an X-directionally movable portion 2006, and the Y-directionally movable portion is disposed so as to be movable in the Y-direction (in the left- and right-hand directions in FIG. 18B) while maintaining a fine clearance from the guide surface by means of the action of the hydrostatic bearing 2009a. Similarly, the X-directionally movable portion 2006 is installed with a plurality of hydrostatic bearings 2009b and is movable in the X-direction (in the left- and right-hand directions in FIG. 18A) while maintaining a fine clearance between the hydrostatic bearings 2009b and the guide surface 2007a. 
A differential exhaust mechanism system is further mounted around the hydrostatic bearings so that no high pressure gases fed to the hydrostatic bearings leak into the inside of the vacuum chamber C. This configuration is shown in FIG. 19. Grooves 2017 and 2018 are disposed doubly around the hydrostatic bearings 2009 and subjected to vacuum discharging always by means of a vacuum line and a vacuum pump (not shown). This configuration allows the Y-directionally movable portion 2005 held in vacuum in a non-contact state to be movable in the Y-direction. The grooves 2017 and 2018 of a double structure are formed on the surface with the hydrostatic bearings 2009 of the movable part 2005 disposed thereon so as to encircle the hydrostatic bearings. The configuration of the hydrostatic bearings is a known one so that a detailed description will be omitted from the explanation that follows.
As is apparent from FIGS. 18A and 18B, the X-directionally movable portion 2006 with the Y-directionally movable portion 2005 loaded thereon is a concave with the top face upwardly open. The X-directionally movable portion 2006 is provided with the hydrostatic bearings and the grooves in substantially the same configuration, and it is held in a non-contact state on a stage table 2007 so as to be movable in the X direction. By combining the movement of the Y-directionally movable portion 2005 with the movement of the X-directionally movable portion 2006, the sample S is transferred horizontally to an optional position with respect to the top end portion of the lens barrel, that is, the charged beam irradiation portion 2002, and it is irradiated at the desired position with charged beams.
Hitherto, a defect inspection apparatus for inspecting a defect of a sample such as a semiconductor wafer or the like has been used in a process for manufacturing semiconductors, the defect inspection apparatus being of a structure so as to inspect the defect of the sample by detecting a secondary electron generated by the irradiation of the sample with a primary electron.
This defect inspection apparatus uses technology designed to automate and render the inspection of defects of a sample more efficient by application of an image recognition technique. This technique is designed to subject pattern image data in a region of inspection on the surface of the sample, obtained by the detection of the secondary electrons, and pre-stored reference image data on the surface of the sample, to a matching operation with a computer and to automatically determine the presence or absence of defects on the sample on the basis of the result of the matching operation.
Nowadays, there is a great demand in the field of manufacturing semiconductors to detect fine defects, as patterns are rendered finer. Under such circumstances, further improvements in precision of recognition are demanded for a defect inspection apparatus utilizing the image recognition technique as described above.
Hitherto, the process for scanning electron beams in the direction parallel to the direction of movement of a sample table and perpendicular thereto while continuously transferring the sample table (JP-A-10-134757, Japanese Patent Application Laid-Open) has been known. Another scanning process is known which involves irradiating the surface of a sample with a primary electron beam diagonally in two-dimensions while projecting in a one-axial direction at equal intervals. It has further been known to perform inspections and so on by dividing electrons from each electron gun into a plurality of electrons and scanning each beam in one direction while continuously moving the sample table in the direction perpendicular to the scanning direction.
As an electron beam apparatus for use in inspecting a defect of a mask pattern for use in manufacturing semiconductor devices or a pattern formed on a semiconductor wafer, there is known an electron beam apparatus of the type that inspects defects of a pattern on the sample, which comprises irradiating an aperture plate having a plurality of apertures with an electron beam emitted from a single electron gun to produce a plurality of images of the apertures, delivering the resulting plural images of the apertures onto a sample, and projecting the secondary electrons emitted from the sample onto the surface of a detector as an image by using a secondary optical system.
The conventional electron beam apparatus of that type, however, fails to take into account the dependency on the angle of the electron beam emitted from the electron gun, and it treats the magnitude of the electron beam as being uniform regardless of the angles of irradiation of the electron beam. In other words, the problem has not been taken into consideration that, in the electron beams emitted from the electron gun, an electron beam having a high magnitude of illuminance is emitted in the direction of the optical axis, however, the illuminance (magnitude) of the electron beam is gradually decreased as the electron beam becomes apart from the optical axis.
Further, there is the problem that the rate of detection of the secondary electron emitted from the sample is high for the secondary electron emitted in the vicinity of the optical axis and that the rate of detection of the secondary electrons drops as the secondary electrons separate from the optical axis. The conventional electron beam apparatus, however, fails to take this problem into consideration.
An electron beam apparatus using a plurality of electron beams is also known, which is used for inspecting a defect in a circuit having a fine circuit pattern, such as a super LSI circuit, or measuring a line width of such a circuit pattern. Such an electron beam apparatus using multi-beams was proposed in order to solve the problem of a conventional electron beam apparatus of the type using one electron beam for forming or inspecting such a fine circuit pattern because such a conventional electron beam apparatus requires a long period of time for processing and fails to gain a sufficient degree of throughput.
In connection with such an electron beam apparatus of the type using multi-beams, there is also known an electron beam apparatus, for example, of the type having a large number of electron emitters arranged in a matrix configuration, which is provided with an open mask between the surface of a sample and the surface of inspection in order to solve the problem that a level of precision in inspection could not be increased because intervals of a detector for detecting reflected electrons or a secondary electrons is extremely narrow so that the reflected electrons or the secondary electrons are likely to invade the detecting region from the adjacent irradiating region.
Moreover, there is known an electron beam apparatus of the type which forms a plurality of electron beams by irradiating a mask with plural apertures with an electron beam emitted from a single electron gun, in order to solve the problem that throughput is decreased due to the fact that scanning requires a long period of time if a defect of a pattern having a line width of approximately 0.1 micron is to be inspected by scanning the pattern on the sample with one electron beam.
In order to perform defect inspection, etc. on a sample having a device pattern having a minimal line width of 0.1 micron or smaller, the ability of a light system is in a limit of inspection from the viewpoint of resolution on the diffraction of light and, therefore it has been proposed that an inspection-evaluation apparatus that utilizes an electron beam. The use of the electron beam, however, has the problem from the viewpoint of productivity because a drastic decrease in throughput is caused, although resolution can be improved. An electron beam apparatus that is modified so as to use multi-beams to improve productivity is also known. More specifically, this known electron beam apparatus is configured in such a manner that the electron beams emitted from a single electron gun are irradiated onto a plurality of apertures and the electron beams passed through the apertures are subjected to scanning of the surface of a sample (hereinafter referred to sometimes as “sample surface”), thereby allowing the secondary electron to be emitted from each image and guiding the secondary electron to each of a plurality of detectors for inspecting the sample.
When a pattern formed on a sample surface such as a semiconductor wafer is to be evaluated with high accuracy by using result of a scanning operation of the electron beam, it is necessary to consider variation in the height of the sample. This is because differences in the height of the sample vary distances between a pattern on the surface of the sample and an objective lens by which the electron beam is to be focused on said pattern, and thereby focusing condition was not satisfied, resulting in deterioration of resolution, which make it impossible to perform an accurate evaluation.
In order to overcome this problem, an electron beam apparatus has been suggested that performs a focusing operation of the electronic optical apparatus in a manner whereby the light is irradiated against the sample surface at a certain angle, the reflected light thereof is utilized to measure the height of the sample, a measurement is fed back to the electronic optical system by which the electron beam is to be focused on the sample, and thereby the current and the voltage applied to the components of the electronic optical system are controlled.
However, in a method for irradiating the light against the sample at a certain angle, an optical component for reflecting the incident light, which is mainly composed of insulating material, should be disposed in a space between the sample surface and a lower surface of the electronic optical system. Thereby, the space between the sample surface and the lower surface of the electronic optical system has to be made wider than is required, while on the other hand, the wider spacing makes such problems as an aberration of the electronic optical system non-negligible. Accordingly, although it is required to perform focusing of the electronic optical system and simultaneously to solve such problems of aberration of the electronic optical system, such method by which both requirements are accomplish has not been suggested.
In addition, since the focusing of the electronic optical system should be performed taking into account not only the distance between the sample surface and the lower surface of the electronic optical system but also a charging condition on the sample surface and a space-charge effect of the electron beam, if parameters relating to the focusing of the electronic optical system are not measured in an electronic optical manner, errors might possibly occur.
Further, there is another problem that, in a case that exciting current of a magnetic lens included in the electronic optical system is regulated to perform the focusing operation, a period from when the exciting current being set to a predetermined value until when a focal length of the electronic optical system is stabilized, namely settling time, must be taken rather longer, and consequently it is difficult to perform the focusing quickly. In another case where exciting voltage of an electrostatic lens is regulated to perform the focusing operation, a high voltage applied to the electrostatic lens shall be varied, which results in the same problem of longer settling time. Furthermore, there is another problem that evaluation by the electron beam results in low throughput.
The present invention has been made with a view solving the various problems described above, and an object of the present invention is to provide an electron beam apparatus capable of performing a focusing operation in an electronic optical system thereof in an electronic optical manner as well as in a short time, and a semiconductor device manufacturing method using the same apparatus.
In a case that defects are to be inspected on a sample having a minimal line width of 0.1 micron or smaller, the inspection by means of a optical light system has a limit due to the resolution due to diffraction of light. Therefore, an inspection-evaluation apparatus using an electron beam has been proposed. The use of the electron beam has improved resolution, however, since it has an extremely decreased throughput, there is a problem from the point of view of productivity. A patent application has been made for an invention relating to an electron beam apparatus for inspecting a sample by using multi-beams with the object to improve productivity, which comprises irradiating a plurality of apertures with electron beams emitted from a single electron gun and scanning the sample with the electron beams passed through the plural apertures, thereby guiding the secondary electron beam generated from each image reciprocally to a detector without causing crosstalk.
A variety of technologies have been reported on apparatuses for observing and evaluating a sample including an insulating material. Among apparatuses installed with such technology, there are known apparatuses installed with a scanning electron microscope, which has a charging detection function for evaluating charging state by measuring beam current of a primary beam, a current absorbed into a sample, amount of electrons reflected from an irradiating apparatus, an amount of secondary electrons emitted, and the like.
Hitherto, there has been known an E×B energy filter for use in conducting an analysis of energy in a field where the electric field is orthogonal to the magnetic field, which allows charged particles to move straight in the direction intersecting with both the electric field and the magnetic field at right angles. This filter allows only the charged particles having a particular degree of energy in the electron beams to travel straight by means that deflection of the electron beams by the electric field is canceled by the deflection of the electron beams by the magnetic field.
As the energy filter of the E×B type, one having the configuration as shown in FIG. 4 is proposed. In FIG. 4, reference numerals 1 and 1′ each denotes a magnetic pole piece held at earth voltage; and reference numerals 2 and 2′ each denote an electrode. A voltage +V is applied to the electrode 2 and a voltage −V is applied to the electrode 2′. These voltages are equal to each other as an absolute value and variable. A charged electron can travel straight in the direction intersecting both the electric field and the magnetic field, that is, in the direction perpendicular to the plane of the drawing.
A stage for accurately positioning a sample in a vacuum atmosphere has been used in an apparatus in which a charged beam such as an electron beam is irradiated onto a surface of a sample such as a semiconductor wafer so as to expose the surface of the sample to a pattern of a semiconductor circuit or the like or so as to inspect a pattern formed on the surface of the sample; it has also been used in another apparatus in which the charged beam is irradiated onto the sample so as to apply an ultra-precise processing thereto.
When said stage is required to be positioned highly accurately, there has been employed a structure in which the stage is supported by a hydrostatic bearing in a non-contact manner. In this case, a vacuum level in a vacuum chamber is maintained by forming a differential exhausting mechanism for exhausting a high pressure gas in an extent of the hydrostatic bearing so that the high pressure gas supplied from the hydrostatic bearing is not directly exhausted into the vacuum chamber.
FIGS. 18A and 18B show one of the examples of such stage according to the prior art. In the stage shown in FIGS. 18A and 18B, a tip portion of a lens barrel 2001 or a charged beam irradiating section 2002 of a charged beam apparatus for generating and irradiating a charged beam against a sample is attached to a housing 8 which makes up a vacuum chamber C. A sample S is detachably held on a sample table 2004. Other structures of the stage of FIGS. 18A and 18B will be described later.
A differential exhausting mechanism is provided surrounding the hydrostatic bearing 2009b so that a high-pressure gas supplied to the hydrostatic bearing does not leak into the vacuum chamber C. This is shown in FIG. 19. Doubled grooves 2017 and 2018 are formed surrounding the hydrostatic bearing 2009b, and are regularly exhausted to vacuum through a vacuum pipe by a vacuum pump, though not shown. Owing to such structure, a Y directionally movable unit 2005 is allowed to move freely in the Y direction in the vacuum atmosphere while supported in non-contact manner.
Those doubled grooves 2017 and 2018 are formed in a plane of the movable unit 2005 in which the hydrostatic bearing 2009b is arranged, so as to circumscribe said hydrostatic bearing. Combining the Y directionally movable unit 5 with an X directionally movable unit 2006 allows the sample S to be moved to any desired position in the horizontal direction relative to the tip portion of the lens barrel or the charged beam irradiating section 2002, so that the charged beam can be irradiated onto a desired location of the sample.
However, the stage including a combination of the hydrostatic bearing and the differential exhausting mechanism as described above has a problem that the overall structure thereof becomes more complex and rather larger in comparison with a stage of hydrostatic bearing type used in the atmospheric air due to the differential exhausting mechanism included therein, resulting in lower reliability as a stage and also in higher cost.
As for methods for compensating for magnification chromatic aberration and rotation chromatic aberration in the electronic optical system, a method using a symmetric magnetic doublet lens is well known. Since no rotation chromatic aberration is generated in the electro static lens system, the magnification chromatic aberration is compensated for by using a doublet lens.
As high integration of semiconductor devices and micro-fabrication of patterns thereof advance, an inspection apparatus with higher resolution and throughput has been desired. In order to inspect a wafer substrate of 100 nm design rules for defects, a resolution corresponding to 100 nm or finer is required, and the increased number of processes resulting from high integration of the device causes an increase in an amount of inspection, which consequently requires higher throughput. In addition, as multi-layer fabrication of the devices has progressed, the inspection apparatus has been further required to have a function for detecting a contact malfunction in a via for interconnecting wiring between layers (i.e., an electrical defect). In the current trend, a defect inspection apparatus of optical method has been typically used, but it is expected that inspection apparatuses using an electron beam may soon be mainstream, substituting for optional inspection apparatuses from the viewpoint of resolution and of inspection performance for contact malfunction. Defect inspection apparatuses using electron beam methods, however, has a weak point that it is inferior to that of optical method in throughput.
Accordingly, an apparatus having higher resolution and throughput and being capable of detecting the electrical defects is desired. It is known that the resolution in the optical inspection apparatus is limited to ½ of the wavelength of the light to be used, and it is about 0.2 micrometer for an exemplary case of a visible light in practice.
On the other hand, in the method using an electron beam, typically a scanning electron beam method (SEM method) has been used, wherein the resolution thereof is 0.1 μm and the inspection time is 8 hours per wafer (20 cm wafer). The electron beam method has the distinctive feature that it can inspect for electrical defects (breaking of wire in the wirings, bad continuity, bad continuity of via); however, the inspection speed (sometime also referred to as the inspection rate) thereof is very low, and so the development of an inspection apparatus with higher inspection speed is desirable.
Generally, since inspection apparatus is expensive and the throughput thereof is rather lower as compared to other processing apparatuses, therefore the inspection apparatus has been used after an important process, for example, after the process of etching, film deposition, CMP (Chemical-mechanical polishing) flattening or the like.
A inspection apparatus of scanning electron beam (SEM) will now be described. In the inspection apparatus of SEM, the electron beam is contracted to be narrower (the diameter of this beam corresponds to the resolution thereof) and this narrowed beam is used to scan a sample so as to irradiate it linearly. On the one hand, moving a stage in the direction normal to the scanning direction allows an observation region to be irradiated by the electron beam as a plane area. The scanning width of the electron beam is typically some 100 μm. Secondary electrons emanating from the sample by the irradiation of said contracted and narrowed electron beam (referred to as the primary electron beam) are detected by a detector (a scintillator plus photo-multiplier (i.e., photoelectron multiplier tube) or a detector of semiconductor type (i.e., a PIN diode type) or the like).
The coordinates for an irradiated location and an amount of the secondary electrons (signal intensity) are combined and formed into an image, which is stored in a storage or displayed on a CRT (a cathode ray tube). The above description demonstrates the principles of the SEM (scanning electron microscope), and defects in a semiconductor wafer (typically made of Si) being processed may be detected from the image obtained in this method. The inspection rate (corresponding to the throughput) depends on the amount of the primary electron beam (the current value), the beam diameter thereof and the speed of response of the detector. The beam diameter of 0.1 μm (which may be considered to be equivalent to the resolution), a current value of 100 nA, and the speed of response of the detector of 100 MHz are currently the highest values, and in the case using those values the inspection rate has been evaluated to be about 8 hours for one wafer having a diameter of 20 cm. This inspection rate, which is much lower compared with the case using light (not greater than 1/20), has been a serious drawback.
On one hand, as a method for improving the inspection rate, which is a drawback of the SEM method, a multi beam SEM using a plurality of electron beams is well known. Although this method can improve the inspection rate by an amount of number of plurality of electron beams, there are other problems associated with this method that since the plurality of electron beams is irradiated from an oblique direction and a plurality of secondary electron beams from the sample is taken out in an oblique direction, only the secondary electrons emanated from the sample at the oblique direction could be captured by the detector, that a shadow is generated on an image, and that since it is difficult to separate respective secondary electrons generated by respective plural electron beams, secondary electron signals are mixed with each other.